So:
Avoid any broadband service with a usage cap.
Be is looking promising at present and has opened up I think 300-500 new exchange areas, the company announced today.
Never use a free service, they are all shit.
If unhappy with your provider, from tomorrow it should be much easier to get the MAC number off of your existing ISP so you can change without loss of access under new Ofcom regs which come into effect Feb 14.
If you can get cable, get cable, it's usually a lot more durable, though more pricey by the Mb - but I have had maybe 12 hours downtime in 5 or 6 years, so I think that's the trade-off, if access is critical.
I signed my mum up to Bulldog. They were not perfect, and I had some issues at first with them, but played some cards and got a better service and a refund.
Check the small print for what you actually get, some providers don't even sent a modem and expect you to get one yourself.
Personally, I feel like the state of the ADSL market is still a nightmare, it is very hard to make value judgements on service.
Watch out for contention rates, capping and the distance from the exchange.
Good luck.